Analysis of accuracy in dose and position in calculations of a treatment planning system for blocked photon fields

Accuracy in dose and position, defined as complementary criteria, was determined for blocked photon field calculations with a pencil beam based treatment planning system. The concept of field accuracy has been defined as a combination of deviations in dose and position. Absolute dose deviations were measured including the deviations in beam output (monitor units), depth doses and dose profiles. Based on this verification work it was concluded that the pencil beam model is capable of calculating dose distributions in blocked photon fields (6 MV and 25 MV) with a high degree of accuracy. The measurements and calculations mostly agreed within 3% or 3 mm, whichever was less, relative to a reference dose at 10 cm depth. Furthermore, the model behaves consistently because a substantial part of the discrepancies found could be ascribed to fundamental assumptions in the model related to electron contamination and primary block transmission. The concept 'field accuracy' allows an easy, quantitative and comprehensive comparison to common quality requirements giving, for example, equal weight to percent dose difference and to millimetre isodose distance. The method offers the possibility to focus on calculation accuracy which excludes some experimental sources of error